In packet based wireless communications networks, it is very important to have a good estimate of the interference level to perform link adaptation. Link adaptation implies to determine at least one of modulation, coding, power and Multiple Input Multiple Output (MIMO) setting, that is/are adapted to the current transmission situation.
The interference is caused by uncoordinated transmissions. This uncoordinated transmission may be caused by other radio base stations or by transmissions controlled by the own radio base station.
An example of such packet based wireless communications networks is Long Term Evolution (LTE) networks. The LTE networks are standardized by 3GPP Long Term Evolution (LTE) which is a project within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard with High Speed Packet Access functionality to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc.
A wireless communication network comprises typically user equipments (UE) 120 wirelessly connected to radio base stations 110 as illustrated in FIG. 1. The radio network architecture may vary between one technology to another. In the LTE, the radio base stations 110 are directly connected to the core network (CN) 100, which is adapted to interconnect the cellular telecommunication system to other systems. In addition, the radio base stations are also connected to each other primarily for exchanging signalling information.
In the existing networks, the link adaptation is performed based on reported and measured CQI (Channel Quality Indicators), and reported and measured received power level, in order to adapt the transmission to the current interference situation. The CQI reports comprise an indication of the signal to noise plus the interference ratio of the reference symbols over the frequency range. In the downlink (DL), the UE measures on predetermined reference signals and reports CQI and power measurements, and provides information on how the channel quality differs over the frequency band. In the uplink (UL), the radio base station measures the quality of the received signal. If the UE also recently has used other parts of the frequency band, a certain awareness of the frequency dependency may be available to the radio base station.
The CQI reports and the received signal level reports are then used to determine the suitable modulation, coding, power and MIMO setting, which is referred to as link adaptation. The link adaptation is done for one UE at a time, and repeated for each sub frame. In LTE networks the subframe is 1 ms.
The channel quality for a certain UE is calculated from the path loss, expected transmit power and experienced interference for this UE. Since the interference for one UE is partially caused by transmissions to and from other UEs, the interference experienced in the past will not be the same as the interference present in the scheduled time frame.
In some situations when a UE is located on or close to the cell border of a first cell and hence experiences poor channel conditions, the UE is very sensitive to interference e.g. caused by other UEs in adjacent cells (UL) or by the radio base station covering adjacent cells to the first cell. Therefore, it is important to have a good knowledge of the interference level for the interference sensitive UE when allocating power in adjacent cells.